1. Field
Disclosed are methods and techniques for obtaining a position fix for a mobile device.
2. Information
Mobile devices can typically obtain a position fix by measuring ranges to three or more terrestrial transmitters (e.g., wireless access points) which are positioned at known locations. Such ranges may be measured, for example, by obtaining a MAC ID address from signals received from such access points and measuring one or more characteristics of signals received from such access points such as, for example, signal strength, round trip delay, just to name a few examples. Such measurements may be viewed as “direct measurements” in that they give information regarding a current position (such as a range to a transmitter fixed at a known location) when obtained.
Typically, measurements of ranges to only three transmitters are sufficient for obtaining a sufficiently accurate estimate of a location of the mobile device. In addition to the use of direct measurements, a mobile device may incorporate “indirect measurements” indicative of relative motion to assist in obtaining an estimate of a current position estimate. Such indirect measurements may include, for example, measurements obtained from signals generated by sensors such as, for example, accelerometers, pedometers, compasses or gyroscopes. Also, in certain environments and applications, movement of a mobile device may be constrained to predetermined areas or paths. In an indoor environment, for example, movement of a mobile device may be constrained to predetermined paths or routes defined according to walls, doorways, entrances, stairways, etc. As such, a current location of a mobile device may be presumed to be constrained by such predetermined areas or paths. However, merely constraining a mobile station's location to a path may not, without additional information, indicate a location of the mobile device with sufficient precision to useful in certain location based applications. Here, the precise location of the mobile device is still uncertain as the location may be anywhere along, or at least proximate to, any portion of a predetermined path.